This invention relates to logic circuits, and more particularly to cascode logic circuits of the type used in integrated circuit devices.
Cascode voltage switch logic circuits are described by Heller et al in ISSCC84, Feb. 2, 1984, p. 16, or by Robert L. Rabe in U.S. Pat. No. 4,833,347, issued May 23, 1989 for "Charge Distribution Resistant Logic Circuits Utilizing True and Complement Input Control Circuits. These are differential CMOS logic arrangements providing the features of no direct current after the latch sets, and low capacitive loading of the inputs. This type of logic reduces the number of devices needed, and thus reduces circuit delays. However, it is found that performance is limited by the set time of the latch. In particular, in a conventional cascode voltage switch logic circuit the voltage one of the output nodes must drop, turning on the pullup on the opposite leg of the differential amplifier, before the voltage on the other output node (and the associated part of the input combinatorial network) may transition high. Second, in the conventional cascode voltage switch logic circuit, all of the positive charge must be supplied by the differential amplifier transistors (the two-up transistors, P-channel in the usual case).
A feature of the present invention is providing a circuit consistent with conventional CMOS logic and processes, that can be used where performance issues are paramount. Particularly, features of high speed and low power dissipation are of importance in the circuit of this invention. Additionally, reducing device count and therefore reducing area in an integrated circuit are objectives.